This invention relates to radiation detector arrangements comprising at least one detector element in the form of a temperature-sensitive resistor having a high positive temperature coefficient, particularly but not exclusively with arrays of such detector elements which detect infrared radiation and which are mounted on a semiconductor circuit substrate comprising circuitry for processing signals from the resistors. The invention also relates to method of detecting incident radiation using such resistor detector elements.
In Chapter 7 of the book entitled "Optical Radiation Detectors", by E. L. Dereniak and D. G. Crowe and published in 1984 by John Wiley & Sons Inc. (U.S.A.) in the Wiley Series in Pure and Applied Optics, various bolometers are described for the detection of infrared radiation. Five types are identified, the most commonly used being the metal, the thermistor and the semiconductor bolometers. All the bolometer types operate on the principle that a temperature change produced by the absorption of radiation causes a change in electrical resistance of the material used to fabricate the bolometer. This change in resistance is detected in known bolometers as a change in voltage across the bolometer element through which a small bias current is applied.
In the metal type, the bolometer material is typically nickel, bismuth or platinum so that the resistor has a positive temperature coefficient and the voltage increases with increase in temperature. In the thermistor type, the material is typically sintered oxides of manganese, cobalt and nickel and has a negative temperature coefficient so decreasing the voltage across the resistor. In the semiconductor type, a material such as doped monocrystalline germanium is used at cryogenic temperatures where it has a high negative temperature coefficient, so also giving a decrease in voltage with increase in temperature.
At page 152 of said book so-called "superconducting bolometers" are mentioned. The temperature-sensitive resistor of this type has a high positive temperature coefficient at a transition in its electrical conductance, from a superconducting state to a more normal resistive state. This is schematically illustrated in FIG. 7-1 of said book. The electrical resistance changes dramatically (by order of magnitude) over the transition temperature range. However, as mentioned at page 152 of said book, this device has not been used extensively as a detector, because of the stringent temperature control that is required, normally at very low cryogenic temperatures.
Thus, for example, for normal operation of a superconducting tin bolometer the operating temperature (about 3.7 K) of the detector element must be stable to within about 10.sup.-5 K. Stabilization of the operating temperature of a superconducting bolometer is considered in some detail in the article of this title by N. A. Pankratov, G. A. Zaytsev and I. A. Khebtov in the Soviet Journal of Optical Technology Vol. 36 (1969) pages 521 to 524. The detector arrangement comprises circuit means (in the form of a bridge circuit) for applying a voltage across the resistor, and temperature-regulation means (in the form of a base cooled by liquid helium) for regulating the temperature of the resistor so as to operate the resistor in the transition region around a temperature of 3.7 K. In this Pankratov et al article the operating point of the bolometer is represented by a simplified heat balance equation of the following general form: EQU G.multidot.(Tb-To)=Wi+Wr (1)
where
Tb is the temperature of the bolometer element PA1 To is the temperature of is base (cooled by liquid helium) PA1 G is the thermal conductance between its cooled base at To and the bolometer element at Tb PA1 Wi is the power dissipated in the bolometer element by the current passed therethrough, and PA1 Wr is the power input to the bolometer element from the incident radiation.
As recognised in this Pankratov et al article, the main cause of instability in these known superconducting bolometers is the variation in temperature of its base which affects the factor G.multidot.(Tb-To). Furthermore, it is difficult to use these bolometers to detect any significant change in the incident radiation power Wr (for example from a hot object in an ambient temperature scene), because such a change may shift the temperature of the bolometer element above the temperature transition region. It is noted in the Pankratov et al article that any instability of the bias current through the bolometer element (i.e. a change in the factor Wi) can be neglected in practice, i.e. Wi is substantially constant.
In addition to the restricted utility due to stringent temperature regulation, the constructions of known superconducting bolometers are not suitable for integration in arrays (either linear arrays or 2-dimensional arrays), and neither are they suitable for mounting on a semiconductor circuit substrate comprising circuitry for processing signals from the bolometers.